Seals are used in many industries. For example, in the art of gas turbine engines, various types of seals are used to limit leakage of gases and fluids. Without these seals, gases and fluids would leak from compartment to compartment. Some of the various types of seals limit leakage through gaps that separate a rotating member and a non-rotating member. One such type of seal is a carbon face seal. Carbon face seals are often used in bearing compartments (e.g., a bearing compartment of a high pressure compressor) to limit leakage of lubricating oil. A carbon face seal includes a non-rotating carbon sealing ring and a rotating mating surface. Springs force the carbon sealing element into contact with the rotating mating surface. The mating surface rotates at the rotational speed of the engine.
Some advanced gas turbine engines subject the carbon face seals to very demanding conditions. For example, a gas turbine engine for a military aircraft often operates at rotational speeds twice as great as that of a gas turbine engine for a commercial aircraft. In addition, military aircraft often undergo maneuvers that introduce significant G forces on the seals. These conditions can cause the carbon sealing ring to wear quickly.
A labyrinth seal is another type of seal used in a gas turbine engine to limit leakage through a gap between a rotating and non-rotating member. A labyrinth seal employs a series of teeth on a rotating member. The teeth are closely spaced to a smooth mating surface on a stationary member. The teeth and the mating surface cooperate to provide a series of constrictions between the rotating and the non-rotating member. The constrictions have the effect of limiting leakage of air from one compartment to another. Leakage has the effect of reducing the efficiency of the engine.
However, because the teeth do not contact the mating surface, a labyrinth seal cannot completely eliminate leakage. Furthermore, the spacing between the teeth and the mating surface must typically be large enough to account for manufacturing tolerances, possible eccentricity of the members and material growth due to temperature and centrifugal loading. Increasing the spacing has the effect of increasing the leakage. The material growth due to temperature typically does not reach an equilibrium until the engine has operated at a steady state condition for 5 to 10 minutes. In addition, in a gas turbine engine for a military aircraft, the spacing must be large enough to account for bending due to aircraft maneuvers that introduce significant G forces.
To put the consequences of leakage through a labyrinth seal into perspective, consider a labyrinth seal used in a turbine rim cavity. Air outside the cavity has a temperature of 3000.degree. F. Purge air having a temperature of 1200.degree. F. is supplied to the cavity in the form of a purge air flow. The purge air flow helps to keep the pressure in the cavity high enough to prevent ingestion of the outside air. The purge air flow to the cavity represents a 1/2 percent of the air flow of the engine and thereby results in a 1/2 percent reduction in the efficiency of the engine. The purge air flow and the resulting reduction in the efficiency of the engine could be reduced if the spacing between the teeth and the mating surface could be reduced.
Seals are also used in a gas turbine engine to limit leakage between an engine case and tips of rotating blades. These seals have a plurality of seal segments mounted to the engine case. The seal segments must be properly spaced from the blade tips for the engine to operate optimally. However, the blades expand and contract during engine operation due to varying operating temperatures and rotational speeds. This results in variations in the spacing between the seal segments and the blade tips.
All of the aforementioned seals have a problem, namely that in order to limit leakage, close spacing between a rotating member and a non-rotating member is required, yet contact between the members causes the seal to wear and there are little or no provision to actively control the spacing.
Efforts have been made to actively control the spacing between the seal segments and the rotating blades. U.S. Pat. No. 5,545,007 discloses an active control system that uses piezoelectric actuators to position the seal segments relative to the blade tips. However, the system requires a separate power supply, in order to drive the actuators.
Scientists and engineers working under the direction of Applicant's assignee have been working to provide other seal designs, and methods for fabricating such seal designs, for use in these and other applications.